Turbine bypass valves are one of the tough purposes in an influence plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to list essentially the most difficult management valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive strain drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many plants are probably to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high costs for reactive maintenance. This article supplies advised strategies of inspection to anticipate and mitigate issues beforehand, and it provides upgrade options ought to a valve need to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in probably the most tough installations inside process crops. These installations generally include cavitating, erosive, corrosive, noisy, high strain, excessive temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process situations; but, they have to reply flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all of the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are applicable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by ensuring solely applicable steam conditions and flows make their approach to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to keep away from tools damage because of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move through the system.
As it operates, a turbine makes use of steam to perform work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’s going to drop the stress, however the exit steam will remain quite superheated, probably destroying downstream equipment. To avoid that scenario, turbine bypass valves both incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a end result, turbine bypass valves face a perfect storm of severe service conditions. While the plant is in operation, these valves must stay tightly closed to keep away from wasting energy. When a turbine trip occurs, the bypass valves must reply immediately, exposing them to rapid temperature modifications and requiring them to cross very high flows at high pressure drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that nearly each turbine bypass valve will in the end fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access locations, are sometimes welded in place, and are typically heavily insulated. As a end result, they’re typically ignored until issues begin to floor. Steam leakage by way of the valve is often the primary symptom seen, however much more important and potentially harmful damage can occur.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can end result in catastrophic failure underneath pressure.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect steel fatigue issues that might be creating but are not but visually apparent. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or approved consultant could additionally be utilized to perform turbine bypass health examine companies (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to identify any developing issues before the tools is compromised. pressure gauge octa and desuperheaters have to be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are additionally threats to trim diaphragm seal , similar to seat surface erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more generally in move down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat area directly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which might comprise water throughout startup conditions and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim injury as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft goods and spray nozzles can often be replaced relatively easily and at minimal cost. Trim components that have to be replaced as a outcome of heavy put on, corresponding to gear toothing, could be quite pricey, particularly if their supply must be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue growing in the valve physique or diffuser, with excessive repair procedures, or perhaps a full valve replacement, required.
Many of the turbine bypass valves presently in service were put in through the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and often have significant indicators of steel fatigue. Others have loads of life left in them however may achieve advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing technology may inject new life into the valve.
Another instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct substitute and evaluate potential improvements. A particularly essential item to contemplate when looking at substitute and improvement alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is good for locations with low overhead clearance, and it places the actuator nearer to the deck or floor for straightforward access, but it creates numerous long-term operational issues. Gravity tends to result in elevated and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the primary option a couple of many years ago for turbine bypass valves. New move up valve designs (right) may be installed without piping modifications in most situations, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement ends in decreased and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is on the market with present horizontal actuator circulate down designs, a move up design can easily meet the present face-to-face requirements of the previous valve, while lowering a number of the put on frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from move down to circulate up, this change may be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a result of the steam is shifting comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves by way of the small holes of the cage, it accelerates considerably, but that power dissipates into the massive body cavity of the valve as an alternative of hanging the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a outcome of they move over the seat and accelerate later as the steam moves through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, but when in comparability with the move down trim shown above (Figure 4 right), the seating floor on the seat ring and plug remain comparatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These photos of flow up trim clearly show the erosive impression of high-velocity steam, but unlike the circulate down trim harm shown beforehand, this valve has no injury to the seating surface and can still shut off tightly. Courtesy: Emerson
The circulate up valve type design still provides the identical management responsiveness and flow capacity, but the new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered equipment that must be carefully specified, installed, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to purchase as they can assist with valve sizing and specifications primarily based on specific working conditions.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been totally inspected, it might be clever to seek the guidance of your turbine bypass valve vendor for help and assist.
Many of those distributors provide full inspection and well being check providers to identify areas the place metallic fatigue and stress cracking are developing. Ideally the valve could be repaired, but if a valve alternative is warranted, the vendor can even present steerage in evaluating bypass valve design styles and choosing the most fitted choice for the particular software.